Send IPC window events for focus and title changes

Patch status: superseded

Patch by Marco Hunsicker

Long description:

This patch fixes ticket #1168 to extend the window IPC event mechanism
to send IPC events for window focus and title changes. The newly added
window events use the same format as the already established "new"
event.
Specifically this patch:
* Moves the ipc_send_window_event() function from src/manage.c into
src/ipc.c and adds an argument for the change property of the event
* Updates src/manage.c to use the new function signature. To ensure
that the "new" event does not send the same event data as the
"focus" event, setting focus now happens after the "new" event
has been sent
* Adds IPC focus event notification to src/x.c. To workaround a problem
during window close when accessing the window name, a function has
been added to query whether a window is actually attached to its parent.
To avoid obsolete focus notification, a new field has been added to
keep track of the focus without any interference by the click handling
* Adds IPC title event notification to src/handlers.c. To avoid
obsolete title notification, a function has been added to determine
whether a window title has actually changed
* Updates the IPC documentation to include the new events
* Updates testcases/t/205-ipc-windows.t to include the "focus" event
in order to ensure the correct event sequence
* Adds two new unit tests, b/testcases/t/219-ipc-window-focus.t and
b/testcases/t/220-ipc-window-title.t to ensure proper "focus" and
"title" events